The present invention relates to a method for the preparation of an oximesilane such as methyl tris(methylethylketoxime) silane, vinyl tris(methylethylketoxime) silane and the like with little coloration.
One of the prior art methods for the synthetic preparation of an oximesilane is a method in which a chlorosilane such as methyl trichlorosilane and the like and an oxime compound such as methyl ketoxime and the like in an amount stoichiometrically equivalent to the chlorosilane are reacted in the presence of a stoichiometrically equivalent amount of an acceptor of the hydrogen chloride produced as a by-product of the reaction (see, for example, Japanese Patent Publication 39-29837). The acid acceptor can be an organic basic compound such as pyridine. In this method, however, it is an indispensable step that the reaction product is separated and isolated from the hydrochloride of the organic base, for example, by filtration and distillation. A serious problem in this distillation process is the danger of explosion so that this method is disadvantageous as an industrial process for the production of an oximesilane.
An alternative method is disclosed in Japanese Patent Publication 1-21834 according to which a chlorosilane compound is reacted with an oxime compound in an amount twice as large as the stoichiometrically equivalent amount. In this method, the excess of the oxime compound over stoichiometry serves as an acceptor of hydrogen chloride.
Further alternatively, Japanese Patent Kokai 63-227592 teaches a continuous method for the preparation of an oximesilane compound in which the hydrogen chloride acceptor is ammonia gas blown into the stoichiometric mixture of a chlorosilane compound and an oxime compound. The amount of the ammonia is larger by 1.04 to 1.46 times than the stoichiometric amount.
A problem common in all of these prior art methods is that the oximesilane product producted by the method is readily colored in yellow or brown when a trace amount or, for example, a few ppm of a heavy metal salt such as iron (III) chloride and the like is present in the reaction mixture. The drawback can of course be solved if the starting materials could be purified to an extremely high purity and the apparatuses used in the reaction could be so designed as not to cause contamination of the reaction mixture with heavy metals although it would be almost hopeless in an industrial process to fully achieve these ideal conditions due to the great technological difficulties and the prohibitingly large costs.
It is known that the above mentioned phenomenon of coloration of the oximesilane compound is very remarkable when the temperature is 90.degree. C. or higher. It is also known that the coloration reaction of an oximesilane compound is greatly accelerated by the presence of unreacted chlorine in the reaction mixture so that the coloration of the oximesilane into brown rapidly proceeds even at a temperature as low as 60.degree. C. or lower. When unreacted chlorine is contained in the reaction mixture, coloration of the oximesilane product proceeds particularly in the step of isolation and purification of the oximesilane from the reaction mixture which usually is performed at an elevated temperature.
In the preparation method of oximesilanes using ammonia as the acceptor of hydrogen chloride, in particular, the reaction hardly reaches completion as compared with the other preparation methods leaving a considerable amount of the unreacted chlorine so that this problem of coloration is very serious. The reason therefor is presumably that the reaction proceeds by involving the gaseous, liquid and solid phases caused as an influence of the ammonium chloride produced as a by-product.
In view of the difficulties above described in obtaining an uncolored product of an oximesilane, it is eagerly desired to develop a method for obtaining an uncolored or little colored oximesilane product by preventing coloration of the reaction mixture, in particular, in the steps of isolation and purification of the oximesilane from the reaction mixture after completion of the reaction.